Various types of brushless d-c motors have been proposed; they are usually supplied from a d-c network which may be derived from a battery of a vehicle, for example at 12 or 24 V, the battery of a telephone central station (48 or 60 V), or a low-voltage supply from a control panel. Usually, only a two-pole or two-terminal d-c network is available, that is, a network which has one ground, chassis, or reference terminal and an active terminal, usually the positive terminal. Two-terminal d-c networks limit the selection of circuits which can be used to drive d-c motors--see, in connection herewith, the article by the inventor hereof in the referenced "asr-digest", and especially FIGS. 1 to 6 thereof.
If a three-terminal d-c network is available, for example a 24 V network having a +12 V terminal, ground or chassis, and a -12 V terminal, then simple circuits can be used as shown, for example, in FIGS. 2 and 5 of the aforementioned literature reference. Few problems arise with induced voltages which occur when the windings of the motor are de-energized. It is, of course, possible to form a three-pole network artificially by connecting two capacitors in series across the network supply to provide an artificial zero or center terminal. The capacitors required, however, must be of substantial size. For a 25 V motor of 4 W power, two capacitors, each of 220 .mu.F, 35 V, are needed. There is a danger of short circuit if, as may occur, both output transistors connecting the respective armature windings of the motor are energized simultaneously. Special protective circuitry is necessary to prevent switching overlaps of the output transistors.
A full-wave bridge circuit can be used with a two-terminal or pole d-c network. This is comparatively complex and requires many components, see FIG. 5 of the referenced literature. It is also possible to connect the motor in a star winding or a two-filamentary, two-pulse motor, with a center tapped winding, see FIG. 3 of the referenced literature. FIG. 6 illustrates a circuit for a four-pulse motor having four armature winding filaments which are star-connected.
A full-wave bridge circuit is efficient with respect to utilization of the wire or copper in the windings and also is capable of good recuperation of inductive energy which arises upon commutation of the windings. It requires, however, a considerable number of components in the electronics, and, there, components which are relatively expensive, for example four output transistors, circuitry for four control signals for the respective output transistors, of which two operate in push-pull. Circuitry is additionally needed to insure that the control signals are clearly temporally separated. If two transistors connected in series across the d-c network are controlled to conduct, even if only for a few microseconds, simultaneously, the resulting short-circuit current would destroy the transistors. Thus, reliability of motor operation requires substantial protective circuitry.
A center tapped winding--see FIG. 3 of the referenced literature--is frequently used in actual structures. Only two output transistors are needed, and the motor operates well with a two-terminal voltage source. There is no danger of short-circuit current even if both output transistors should have overlapping conduction times during short periods. Only the efficiency of operation is affected. It is difficult, however, to recuperate the energy released upon commutation and high voltage peaks may occur in the winding filaments which are disconnected. Such voltage peaks can be suppressed only with Zener diodes or with R/C networks, in which the R/C networks are less effective than the Zener diodes, although cheaper. Alternatively, it is possible to avoid such voltage peaks by soft or gradual disconnection or interruption of current flow through the armature windings; this, however, requires additional circuitry expense--see, for example, U.S. Pat. No. 4,099,104.
An improvement can be obtained--see U.S. Pat. No. 3,840,761, and particularly FIG. 20 thereof--by forming the filaments of the armature windings as dual or parallel filaments--see also FIG. 3 of the referenced literature. Making the windings as dual filaments, in which the filaments of the windings are positioned essentially in parallel, results in tight coupling of the two filaments of the windings. The voltage peaks are then avoided. It has been found, however, that such a motor has a disadvantage which is not immediately apparent: A square-wave voltage is applied over the entire length of both winding filaments with the full amplitude of the operating voltage, that is, in case of an operating voltage of the motor of 24 V, the square-wave voltage between the filaments will be about 48 V. By definition, a square-wave voltage has steep flanks. The relatively high voltages between the individual strands or filaments of the windings, and particularly the steep flanks of the voltage pulse, substantially stress the insulation of the winding and cause deterioration of the insulation as time progresses. Thus, this type of winding is restricted to operation with low operating voltages, that is, in the order of between 12 to 24 V, since operation with higher voltage places excessive stress on the insulation and impairs the reliability. If the motor is to operate under condition where reliability of operation is important, or with higher operating voltages, double-insulated wire is needed.